This invention relates to a method and apparatus for producing fuel gas.
Most of the world's iron production comes from blast furnaces. The primary function of a blast furnace is to reduce ore to iron. A charge comprising iron ore, coke and fluxing ingredients is introduced into the furnace through its top and forms a bed. A blast of pre-heated air is used to burn coke to form carbon monoxide. Carbon monoxide reduces the iron ore to iron. The heat liberated by the combustion of the coke is used to melt the iron produced. Iron and slag are removed as molten products at the bottom of the furnace. One disadvantage of this process is that it is necessary first to convert coal to coke. This operation is performed in a coke oven using a reducing atmosphere. Coke ovens are both expensive to run and produce residues which present problems when it comes to their disposal in an environmentally acceptable manner.
It is has therefore been proposed to produce iron from iron ore using coal directly. A two stage furnace is typically used. In an upper stage iron ore is reduced to iron by reaction with a reducing gas. The resulting iron is sent to a second stage in which it is melted. The second stage also serves to gasify coal via partial oxidation reactions so as to yield a reducing gas for use in the first stage. Such "direct reduction" processes have now come into commercial use.
One feature of direct reduction processes is that they generally produce reducing gas at a rate greater than that required for the reduction of the iron oxide. Since direct reduction processes are often operated at sites that are relatively remote from other industrial activity, it is typically uneconomic to collect the excess reducing gas and employ it as an industrial fuel at other sites. It is therefore often desirable to employ the fuel gas on site in the generation of electrical power. It has therefore been proposed to feed the fuel to a gas turbine forming part of a combined cycle power generator in which combustion of the fuel is performed at elevated pressure, the combustion gases are expanded in an expansion turbine, the hot exhaust gases from the expansion gases are heat exchanged with water or steam so as to form superheated steam, and the superheated steam is expanded in a steam turbine, with the expansion turbine and the steam turbine both being employed to drive alternators so as to generate electrical power. A disadvantage of this arrangement is that, in practice, the rate of production of the excess reducing gas tends to vary with changes in demand for iron from the direct reduction process. Such a variable flow of fuel gas is particularly disadvantageous in the generation of electrical power.
It is therefore an aim of the present invention to provide a method and apparatus which ameliorates the above difficulty.
EP-A-0 657 550 relates to a method for producing iron, comprising the steps of reducing iron ore to iron by reaction of a reducing gas in a first stage and melting the iron and gasifying a solid carbonaceous material both in a second stage, the gasification of the solid carbonaceous material yielding a reducing gas for use in the first stage, characterised in that at least 25% by weight preferably all of the carbonaceous material comprises particulate cold char formed by the partial oxidation of coal in a reactor separate from the first and second stages. This method enables a more thermally efficient process to be provided. In effect, the gasification of the coal is conducted in two separate steps, one being initial conversion of the coal to a char in the reactor, and the other being gasification of the char in the second stage. The rate at which excess reducing gas is produced is substantially unaffected by the addition of the reactor and therefore this method does not provide a solution to the above described problem.